I know this sound strange but I have an FNMI 20 barrel that has a front hand guard cup that is moving WAY too much.  Never seen one move so much that with the handguards on it.  I am getting 3/32" of movement.  Rather than removing the FSB would a small MIG tack weld on the inside or silver solder the cup to the barrel work?   I know it does not affect function but it's driving me nuts.

Welding a loose handguard cap seems a tad extreme.

It move too much, but I hear you.   Either way I am going to have it done.  Got the barrel at a good price.

Might seem as if the tapper pin grooves are mis-located.  Probably because the gas tube hole is also in the wrong spot.
Or the abutment for the end cap is mis-located.  May be why you got such a deal.

Why is removing the FSB such a big deal?  Do you have the right pin punch, hammer and a way of holding the barrel secure?

Removing the FSB and shimming your hand guard cap with an appropriate washer would be much, much, much better than tacking it with a MIG welder - even if the barrel is cheap.

I am not an expert on the metallurgy of 4140 or 4150 steel, but I would be afraid that the metal discontinuity, untempered martensite in the HAZ (heat affected zone), and stress in the steel from weld shrinkage could conceivably start a crack after so many rounds fired.

On the other hand, it might turn out just fine - but why would you really want to do this?

Something along the line of JB Weld or a good high temp epoxy may be a better fix.

Thanks for the tips.  

The Gas tube lines up fine the FSB is timed perfectly on the barrel.  Not sure what's up with the handguard cap but I will try the other options.

Most likely the barrel diameter is undercut where the handguard caps rests. I have personally taken an FNMI 20" barrel apart that had the same problem as yours. We used a thin o-ring in there and put it back together. Tightened it right up, but I think it was a reject barrel and somehow ended up at a gun show. Even with the loose cap, I'd still like to have one for myself. Buddy only paid $100 for it. All it needed was handguards.

Is it front to back movement, side to side, or rotation? If front to back, use a hose clamp.

double tap